The M2-1 protein of the important pathogen human respiratory syncytial virus is a transcription antiterminator that is essential for viral gene expression. We present the X-ray crystal structure of full-length M2-1 protein in its native tetrameric form at a resolution of 2.52 Å. The structure reveals M2-1 forms a disk-like assembly with tetramerisation driven by a long helix forming a four-helix bundle at its center, further stabilised by contact between the zinc finger and adjacent protomers. The tetramerisation helix is linked to a core domain responsible for RNA binding activity by a flexible loop on which lie two functionally critical serine residues, 58 and 61, that are phosphorylated during infection. The identity of these residues was confirmed by mass spectrometric analysis of M2-1 protein expressed in baculovirus-assisted insect cell culture. The crystal structure of a phosphomimetic M2-1 variant, S58DS61D revealed altered charge density surrounding this flexible loop, although loop position was unaffected. Structure guided mutagenesis identified residues that contributed to RNA binding and antitermination activity, revealing a strong correlation between these two activities, and further defining the role of phosphorylation in M2-1 antitermination activity. The data presented here identify surfaces critical for M2-1 function that may be targeted by anti-viral compounds, and allow us to propose a possible model for M2-1 function during respiratory syncytial virus transcription.